This invention relates to DC to DC converters and, more particularly, to a DC to DC converter in which the output voltage may be regulated against variations in input voltage and output current. A DC to DC converter, as is known, converts the DC voltage provided by a DC source at an input to a voltage level different from that of the DC source applied at an output. By way of example, this type of voltage-converting device finds application as part of traveling wave tube amplifier systems where a low voltage DC source must be converted to one or more high voltage DC sources suitable to operate the electrodes of a traveling wave tube.
One design of converter which has been heretofore employed in such a traveling wave tube amplifier system uses a transformer having a primary and a center tapped secondary, with the secondary winding ends connected through rectifiers and connected in series with a large inductor for connection to one end of a load between the inductor and the secondary winding tap and with a pair of switches, such as transistor switches, connected in series circuit to alternate ends of the input, with one end of the primary winding connected to the juncture of the two transistor switches and the other end of the primary winding connected to the juncture of a series connected pair of capacitors connected across the input, with means responsive to the low voltage for varying the periodicity of control pulses alternately applied to the transistors. A sophisticated example of this type of circuit of which we have knowledge appears in U.S. Pat. No. 3,745,440 to Lord, owned by the assignee of the present invention. In the aforedescribed circuit, to an approximation, the current into the transformer primary is of a ramplike waveform; the current increases linearly in level with lapse of time to a predetermined maximum and then abruptly terminates. The output voltage is controlled by means varying the ratio of the on time of the transistor switches to the time of a half cycle. It is recognized that the inductor used in such device is bulky and heavy. Moreover, as a result of the ramp shaped waveform of the primary current, the peak current through the transistor switches is at least twice the average current. This limits the peak power which can safely be handled by the device because of current and voltage limitations on presently available transistors. Additionally, the transistor must switch from a current-conducting or "on" state to a noncurrent conducting or "off" state at a time when its collector current is at the highest level. Thus a good deal of power is dissipated in the transistors during the interval in which the transistor is turned off. Furthermore, an undesirable side-effect of the fast rise and fall of current in the operation of the aforedescribed prior art circuit, the converter generates levels of high frequency electromagnetic energy that could cause interference. By way of further background, there has been made known to us a voltage converter design presented in U.S. Pat. No. 3,582,754 to Hoffman et al. Hoffman discloses a DC to DC converter that is self-oscillatory but which does not contain any structure to vary the width or the duty cycle of his primary currents. The converter in the Hoffman patent shows the use of a capacitor which in conjunction with an inductance produces half sinusoids of current in a transformer primary and in which the leakage reactance existing between the primary and secondary winding is used as the inductance. And lastly, the Hoffman converter shows the use of a pair of series connected diodes to clamp the peak voltage excursion at one location, not directly across the capacitor, within the circuit as is brought out in the patent. The Hoffman circuit is intrinsically nonregulating and line voltage regulation is achieved by the use of "add-on" devices. As becomes apparent hereinafter, although he circuit of Hoffman contains features which are similar to the structure found in the present invention, the arrangement and cooperation of elements differs and achieves a different result.
The present invention has as an object the provision of a high power DC to DC converter that is less bulky, more efficient and capable of handling greater amounts of power than previous designs actually employed and which, moreover, can be regulated over a wide range of input voltages and load conditions.
It is a further object of the invention to provide a high power DC to DC converter particularly adapted for use in traveling wave tube amplifier circuits without the necessity of a heavy bulky inductor and which incorporates a regulating feature of relatively uncomplicated structure.